With the development of the wavelength division system, requirements on the flexibility of the system have increased. WSS can attenuate and switch any wavelength to any port and provides a technical platform necessary for flexible integration of optical networks and has gradually gained acceptance.
As the core equipment of optical transfer networks and reconfigurable optical add-drop multiplexers (referred to as ROADM), the WSS offers more convenience in developing new business and reducing operation costs for network operation. Firstly, when providing business at the wavelength level for large business clients, with the use of ROADM nodes, it is possible to configure the network system remotely through network management systems. With such remote configuration, it greatly facilitates the development of new business areas and enables faster responses to clients' changing demands. Secondly, the application of ROADM facilitates network planning and reduces the operating expenses. For rapid and unpredictable business clients, ROADM can provide node reconstruction abilities to make it possible that the network based on Dense Wavelength Division Multiplexing (referred to as DWDM) can also now conveniently reconstructed, thereby greatly reducing the requirement on network planning. In addition, the ability to handle network emergencies is enhanced, which in turn improves the overall efficiency of the entire network. Moreover, the application of ROADM is convenient for maintenance and reduces maintenance costs. If ROADM is adopted, most daily operations and maintenance, including setups for new clients and cable line adjustments can be executed by network management without manual operation, which in turn improves work efficiency and reduces maintenance costs. As a new generation of product, the WSS makes optical network layer realize full automation at wavelength level.
Chinese patent CN1831574A has disclosed a wavelength selective switch (WSS), which includes a spectrum dispersing part, a movable reflector, movable reflector drive and reflected light monitor. The WDM (Wavelength Division Multiplexing) optical signal at the WSS input port is divided into individual wavelength channels by the spectrum, and the wavelength channel along different directions can be reflected by the corresponding movable reflector. The angle of the reflecting surface at each movable reflector corresponds to the position of the output port set in the output address of the wavelength channel to be reflected. The wavelength channel arriving at the target output port is reflected by the reflector on the output port, and the reflected light advances downwards along with the direction opposite to the previous optical path and is then returned to the input port. In addition, this WSS has the function of monitoring the reflected light. However, this patent can only realizes the function of optical path selection switch through rotating the reflector, but cannot perform the function of attenuation and hitless switching.
U.S. Pat. No. 7,092,599B2, as shown as FIG. 2, disclosed another type of wavelength selective switch, which was based on grating and LCD technology. However, due to the thermal characteristics of LCD, it is necessary to use a temperature control. The polarization performance is also poor, resulting in a slow response time. Therefore, its application is rather limited.
In recent years, Joseph E Davis of Capella Company has filed a number of patent applications related to WSS. The basic principle has not significantly changed, but new functions are continuously added to achieve better optical specifications. The WSS described in his latest patent application, 2006/0228071, is similar to the earlier patents, including a fiber collimator array, diffraction grating, optical lens, reflector array, and ¼ wave plate. Incident light signal after the split by diffraction grating is separated to individual wavelength channel light signals. The discrete channel optical signals are transmitted to a two-dimension reflector array. The two-dimension reflector array realizes signal deflection (wavelength selective switch) through rotation in one direction, and then realizes attenuation of each channel signal through rotation in the other direction. It teaches a method that achieves optical path switch selection through rotating the two-dimension reflector in one direction, and realizes attenuation and hitless switching through rotating the two-dimension reflector in the opposite direction.
For guaranteeing working bandwidth required for WSS, the smaller the gap between Micro-Electro-Mechanical Systems (MEMS) reflectors is, the better; that is to say, the bigger the fill factor is, the better. Generally speaking, a two-dimensional MEMS reflector has one more rotating axis than a one-dimensional MEMS reflector, and thus its fill factor is relatively smaller. The patent (U.S. Pat. No. 6,934,439 B2) issued to JDSU Company is specifically directed to a solution to overcome the smallness of the fill factor by designing a “plano”-type MEMS two-dimension reflector, as shown in FIG. 1 in the patent.
In summary, the existing WSS based on the two-dimension MEMS reflector technology has two issues: (1) high cost and (2) small fill factor. In addition, it is very difficult to design and manufacture two-dimension MEMS reflectors. The passing rate of finished products is low. Presently, only a few American and European companies can supply the WSS based on two-dimension MEMS reflector technology. Moreover, the cost for two-dimension MEMS reflectors is understandably high, leading to high prices of WSS.